Proximity cards are generally known. Such cards typically include a resonant circuit and a processor coupled to the resonant circuit.
A proximity card may be used in conjunction with a card reader that wirelessly transmits a radio frequency signal to the proximity card at a frequency that causes the resonant circuit of the proximity card to resonate. This resonance within the resonant circuit charges a power capacitor of the proximity card.
The capacitor of the proximity card provides power to the processor of the proximity card. Upon receiving power from the capacitor, the processor wirelessly transmits a data signal to the card reader.
While proximity cards work well, they may be compromised with the appropriate equipment. For example, a thief may walk through a crowd with a portable transceiver that transmits a signal at a number of different frequencies. As the portable transceiver enters a resonant range of any nearby proximity cards carried by authorized users, a nearby proximity card may transmit a response. The portable transceiver carried by the thief copies the data of the response transmitted by the nearby proximity card. Once the thief knows the resonant frequency and response, the thief is able to prepare duplicates of the proximity card. Accordingly, a need exists for better methods of protecting the data saved on proximity cards.